A typical hydrocarbon synthesis process, such as the Fischer-Tropsch process, involves the hydrogenation of CO in the presence of Group VIII metals such as Fe, Co, and Ru. The products formed from this reaction are gaseous, liquid and waxy hydrocarbons as well as oxygenates that include, inter alia, olefins and paraffins. The carbon distribution of these products is described by the Anderson-Schulz-Flory distribution.
Fused iron catalysts are known in the prior art to be used in Fischer-Tropsch synthesis. They are generally used in fluidized bed systems which operate at high temperatures. Such a fluidized bed system may include a fixed fluidized bed reactor.
Processes of this character, wherein fluidized solids are contacted with gases, have a number of inherent and important advantages. For example, intimate contact between the gases and the fluid subdivided solids is secured. It is also possible to maintain a substantially uniform temperature throughout the bed as a result of the extremely rapid transfer of heat from one section of the bed to the other because of the rapid circulation of the fluid subdivided solids. Furthermore, due to the rapid transfer of heat between the solids under these conditions, it is possible to readily add or remove heat from the system at an extremely rapid rate.
In these fluidized reactions the subdivided solids or catalysts usually have a particle size in the range of from about 1 to 200 microns. These particles are suspended in a fluid ebullient state by means of the up flowing suspending gases, the velocity of which may vary.
The fused catalyst have a high mechanical strength, which is higher than that of precipitated catalysts. The strength of the catalysts is essential, as there is rapid mixing in a fluidized bed system. Fluidized beds are also operated at high temperatures (280-350° C.) so that the products are all gaseous. Unfortunately, carbon formation and deposition on the catalyst also occur at these elevated temperatures. The deposited carbon originates from the CO in the synthesis gas. This deposition causes the catalyst particle to swell and disintegrate and eventually requires the replacement of the catalyst as the swelling particle and additional fine material creates an expansion of the fluidized bed. Temperature control and the control of the entire synthesis reaction substantially deteriorates due to poor catalyst fluidization.
A number of workers in this field have proposed various methods of improving the fluidizing characteristics of the solid iron catalyst in view of its affinity to form carbon during the synthesis process. For example in U.S. Pat. No. 2,459,444 the invention described therein claims a method of improving the fluidizing characteristics of the powdered iron catalyst for the synthesis of hydrocarbons by mixing with the iron a quantity of a coarser or larger particle size powdered inert material such as silica gel. Whereas, on the other hand, U.S. Pat. No. 2,471,913 proposes the use of an inert solid siliceous diluent in the synthesis zone “in order to maintain fluidity of the catalyst”.
The fused iron catalyst can be prepared from low impurity iron sources, for example, presently Sasol uses millscale from a steelworks to prepare its Synthol catalyst. The disadvantage of using such a material is that the supply is dependent on the throughput of the steelworks and the impurity levels in the millscale are not always consistent.
One disclosure in the prior art: U.S. Pat. No. 2,758,128, relates to a carrier free iron catalyst which is prepared by means of forward precipitation and is suitable for hydrogenation of carbon monoxide with the production of a high yield of low boiling gasoline-like hydrocarbons.
The technique of forward precipitation described in this patent entailed addition of an iron salt solution (also including copper and lime) to a boiling soda solution. The precipitated catalyst was impregnated with a promotor and was then dried at 105° C., crushed and reduced. The reduced catalyst so formed had a desired large inner surface of 110 to 180 m2/per gram of iron which is achieved mainly by using the precipitation method. The patent further reveals that the catalyst may be used in the carbon monoxide hydrogenation with the use of “fixed beds” as well as in hydrocarbon synthesis operated with the catalyst suspended in liquid media (or “slurry process”). However, it is specifically stated therein that the application of a catalyst prepared according to that particular invention in the “fluidized process” is not possible.
Precipitated iron catalysts are generally known to be not suitable for high temperature operation due to their high specific activity related to the high surface areas and large pore volumes. The strength of precipitated catalysts generally does not match the strength of fused catalysts.
In U.S. Pat. No. 4,340,503, a method of preparation of a supported iron catalyst is described wherein a silicate support substantially free of aluminum is impregnated with iron and potassium and the material is capable of converting synthesis gas to C2-C4 olefins. The catalyst is said to be suitable for operation in a fluidized bed reactor as would be expected for a supported impregnated material.
Certain components of a fused iron catalyst which are inherited from the metal parent ore are not desirable in certain instances, for example, Al2O3. Whenever Al2O3 is in excess of certain amounts, it provides too much acidity to the catalyst and therefore the synthesis process results in an increased production of the paraffins.
The inventors of the present invention have now developed a precipitated iron catalyst which is capable of hydrogenating carbon monoxide in a fluidized bed process. Such an unsupported precipitated catalytic material should ideally still be suitable to withstand the turbulent dynamics of a fluidized bed reactor without negatively affecting its performance, which should be comparable to that of a fused iron catalyst, but with reduced affinity for carbon formation during the synthesis process. Such a precipitated catalytic material should ideally comprise none or predetermined minimal amounts of impurities unlike the fused iron catalyst.